Resistance thermometer



F. G. KELLY RESISTANCE THERMOMETER Filed July 25, 1940 Jan. 5, 1943.

Gttorneg 'Patented Jan. 5, 1943 -UNITED STATES PATENT OFFICE RESISTANCE THERMOMETER. Frederick G. Kelly, West Orange, N. J., assignor to Thomas A. Edison,

Incorporated, West Orange, N. J., a corporation of New Jersey Application July 23, 1940, Serial No. 346,892

8 Claims.

This invention relates to resistance thermometers, and more particularly to the resistor bulbs thereof which are adapted to be immersed in a fluid (gaseous or liquid) medium and to respond to changes of the temperature of the medium with corresponding changes in their electrical reslstances. Typically these bulbs are intended to be supported in the wall of an enclosure within which is the fluid medium Whosetemperature is to be measured, for example in the wall of an' engine manifold or crank-case, the sensitive or resistance-element portion proper of the bulb extending into the medium for thermal response thereto. The variations in the resistance of the element proper are translated into a suitable indication, for example visual, by any appropriproper to the interior of the shell.

ate means such for example as an electricalV A bridge circuit of which the element may form one arm.

The wide difference of temperature which may exist, between the wall in which the bulb is supported and the fluidmedium in which the sensitive portion of the bulb is immersed, tends to result in substantial error in the indication of the temperature of the medium. ConstructionsA specifically intended to minimize this tendency,

however, themselves tend markedly to increase the time lag of the device, or the delay characterizing its response to changes in the temperature of the medium. It is an object of my invention to minimize simultaneously the error and the time lag both abovementioned. And it is an object to provide a resistance thermometer bulb, adapted for such as abovementioned, whose operation is characterized by smallness of error and of time lag jointly.

For strength and ease yof construction it is preferable that the well or shell of the bulb, which extends into the medium and within which the resistance element proper is contained, be made of metal, as distinguished from glass, ceramic, plastic or other non-metallic material. It is an object of my invention to fuliil the objects stated above in a device employing a metallic well or shell.

In achieving the singleresult of small error with metal-shell devices. recourse has at times l been hadl to the expedient of thermally insulating the `shell from the base of the bulb-l. e., from the portion immediately secured in the enclosure wall, and thus exposed to the temperature of the wall (and usually also to at least some extent to the temperature obtaining outside of the enclosure). This expedient is a troublesome one, particularly when, as is other- It is another object to provide a particularly improved combination of insulation for the wire of the resistance element, and binding medium for holdingthe same to the shell, to result simultaneously in good heat conductivity, mechanical strength, and chemical imperviousness to deterioration.

It is another object to provide for the resistance element a wire which has both particularly favorable mechanical properties for the specific use, and a high sensitivity in respect of resistance Achange with temperature.

It is another object to fulfil priorly stated objects in a device adapted for the measurement of very high temperatures without harmful effect on the device.

It is another object to provide a resistance thermometer bulb particularly well adapted to withstand heavy vibratiomas in airplane service.

It is another object to provide a generally improved resistance thermometer bulb.

Other and allied objects will more fully appear l from the following description and the appended claims.

In the description reference is had to the accompanying drawing, in which:

Figure 1 is a cross-sectional view taken axially through a resistance thermometer bulb in which my invention is embodied;

Figure 2 is a cross-sectional view taken along the line 2 2 of Figure 1; A

Figure 3 is a fractional cross-sectional view, on an enlarged scale, taken along the line 3-3 of Figure 2;

Figure 4 is an end view of the device of Figure 1, the line I--I in Figure 4 indicating the plane on which Figure 1 is taken; and

Figure 5 is a schematic view of a typical circuit in which the device of Figures 1 through 4 may be employed. y

Reference being had to the drawing, there will be seen designated as l the base of the bulb; this base may be formed of any convenient metal,

^ circuit such as shown in Figure 5.

although I have preferred to employ for it a. material similar to` that mentioned below for the shell il. The central or largest-diameter portion 3of the base may be of hexagonal shape (as seen injigure 4) toadapt it for manipulation by a'wrench in the mounting of the bulb. The base may have an externally threaded portion 2 of-somewhat reduced diameter extending for a distance from the portion 3; and it will be understood that in thel mounting of the bulb the base portion 2 may be screwed into a wall (such as schematically indicated asW in Figure l) the medium whose temperature is to be measured vbeing on one side of the wall and the central base portion 3 being exposed on the other side of the wall.

The base i maybe centrally apertured-for example, through its portion 2 and the immediately adjacent part of portion 3, for a diameter of about or e. little V:more than l/g inch. Extending into and secured within this aperturing is one end portion of a shell il, which may extend from the base portion 2 to have its free end for example almost 2 inches therefrom. This free end of the shell il may be closed by a disc l2 integral with or welded to the shell, the shell and disc together constituting a well which is im- Vmersed in the medium whose temperature is to be measured. Within this well is contained the resistance element proper as hereinafter more specidcally described.

- The base i of the bulb may have, extending from the ,central portion' 3 outwardly (or oppositely to the portion 2), a reduced-diameter externally threaded portion 4; and the central aperturing of the base may be extended through the outer part of the base portion 3 and through this portionv 4,-preferably with an increased diameter. Onto the portion I may be threaded a sleeve 5. This sleeve may be provided with an internal shoulder 5a; and this shoulder may be employed to clamp, against the outer or free end of the base portion 4, a thick disc 8 of insulation-for example, of the ceramic material commonly known as Isolantitef Preferably this clamping will lbe through'the medium of gaskets l of soft metal such as aluminum. one of thet gaskets being disposed between the shoulder 5a and the disc 8, and the other between that disc and the end of the base portion 4. Extending through and anchored within the disc 6. and extending outwardly therefrom within the outer portion of the sleeve 5, may be a pair of contact pins 8. The construction is adapted for the electrical contacting of pins 8 by a female plug-type connector (not shown) to be inserted within the outer portion of the sleeve 5; and the outer end of the sleeve may be threaded externally to receive the swivelling clamp or housing (not shown) commonly employed in association with such a connector. The connector may be employed with an armored cable (not shown) for connecting the pins B-and thus the resistance element proper, which will hereinafter be seen to be connected thereacross-into an indicating Attention may now be directed to the resistance element proper, and to the relationships between it and other portions ofthe bulb. This elementis designated in the drawing as I3; it is mounted within the well of the bulb in a position which, as is common in devices of this character, preferably is substantially immediately adjacent to the internal wall of the shell and,

longitudinally, is relatively nearer the free end of the shell.

As has been touched on above, the diierence between the temperature of the wall (and/or the temperature external of the wall) and hence of the base of the device, and the temperature of the medium to be measured, may-introduce substantial error into the temperature indications provided by the device; this is by reason of the tendency for the resistance element' i3 to assume a temperature intermediate, between that oi the mediumV and that of the base. Attempts to employ devices of this type wherein such a substantiai error occurs, by allowing for a. fixed-percentage error in the calibration of the indicating instrument, are not particularly satisfactory at best; and they are futile if the same or similar instruments are to be used for measurements of dierent media (for example, air in one case, oil in another) in view of the great dependence of the degree of error on the medium. On the other hand, as has also been ltouched on above, not all expedients which would minimize the tendency to error are useful, as most of them tend seriously to increase the time lag with which the temperature of the element i3 will follow changes in the temperature of the medium.

vI have found that for simultaneous achieve ment of small error and small time lag, three primary conditions must be jointly fulfilled: (a) the thermal conduction from element to base (i. e., in a direction longitudinal of the device) must be small; (b) the thermal capacity of the shell portion most immediately surrounding the element must be small; and (c) all further thermal capacity effectively associated with the element must be small. Condition (c), and condition (a) insofar as it concerns elements other than the shell, I jointly meet by causing the shell to constitute a vmajor fraction (for example, at least or 3A) of the entire thermal capacity effectively associated with the element, and to constitute at least a similar vfraction of, and preferably substantially the entire, admittance to heat ow between element and base. Condition (b), and condition (a) insofar as it concerns the shell, I have found cannot be satisfactorily met with glass, ceramic or other non-metallic materials; for whilejthey facilitate compliancewith (a) they are unsatisfactory, when employed with wall thicknesses suicient for mechanical rugge'dness, in complying with (b). On the other hand I have found that these two conditions cannot be satisfactorily met with the lmetals commonly employed for bulb-shells; for while in practicably thin wall thicknesses they facilitate thermal capacity associated with the element and the heat conduction from element to base, as abovementioned, and further is constructed withra very thin wall of a metallic alloy having a thermal conductivity of the order of .07 calorie per degree C. per square cm. per cm. per second,

or less, then surprisingly small error and time fractions of the entire thermal capacity eil'ectively associated with the element I3l and of the total admittance to heat now between element and base, the elementfl is'formedI as a cylindrical winding, of insulated resistance wire, substantial- Vly fitting the bore of the'shell II, and internally supported only by a thin flat plate or card I8 of insulating material such as mica. This card I8 will be seen to extend for substantially the entire length of the shell Il, and permissibly toextend slightly from the end of the shell within the base I. The extremely low heat conductivity of such material, and the small thermal capacity- (of its Vportion within and near the element I3) at- I4. The winding may be formed in place on the card with the aid of an axially split mandrel; the ends of the wire I4 may be fed through holes such as I8b in ,the card; and from points Ila of electrical junction with those ends', conductive leads I5 may be fed through appropriate anchoring holes `I8c in the card up into thebore of the base I, for final electrical connection with the respective pins 8. After the formation of the winding and its equipment with the anchored leads I5, the card and winding and mandrel may be slipped together into the well, and the mandrel then removed.

Before the abovementioned insertion of the card I8, the shell II may bev tightly fitted within the bore of the base I, and will preferably be silver-soldered or otherwise intimately secured to the base. The alloy of low thermal conductivity which I have preferred to employ for the shell I I is stainless steel (for example, 18-8", containing 18% chromium and 8% nickel) which has a thermal conductivity of approximately .052 calorie per degree C. per square cm. per cm. per secondalthough other alloys having a thermal conductivity of the order of .07 (in the same units) or less may satisfactorily be employed. vI have employed the stainless steel material abovementioned in a wall thickness of the order of .012 inch. With this thickness, and withv the winding or element I3 placed at an average distance of about 1% inches from the base I (as I have placed it), the thermal conduction through the shell from element to base is only about .0011 calorie per degree C. per second. By way of comparison of these figures with thosefor commonly employed metals, it may be pointed out that they are less than 1,(,th of those for pure aluminum, and

of the order of Vath of these for the most common I aluminum alloys. Observations have shown the errors with my device to be of the general order of only 6% (o f the temperature difference between mediiun and base) in measurement of an air medium and .045% in measurement of an oil medium, as contrasted with about 33% and 4%. for air and oil respectively, with an otherwise 'similar device having a shell of one of these common aluminum alloys. At the same time my device, on sudden immersionl in a new.

medium temperature, responded with 90% of its ultimate temperature change indication at the expiration of a lag ofonlyf16 seconds-of which tests have shown the element I3 itself to be responsible for about 2 seconds, the card I8 to be responsible for about 3 seconds. and the shell v'IIS II to be responsible for about 1l seconds (or over 3/4 of the total lag caused by card and shell).

It will of course be understood that the wall thickness and/ or thermal conductivity of the shell may be varied to some extent from the specific examples given above; but in choosing theml prefer to keep the thermal conduction through the shell between element and base at least as low as approximately .0015 calorie per degree C. per second.

'Both for imperviousness to the effects of high temperature, and for its special adaption to the binding medium hereinafter mentioned, I prefer to employ a resistance wire I4 which is insulated with a glass fibre Wrapping-i. e., with a single or preferably a double Serving of glass fibre over the wire. To secure a particularly intimate thermal bond of the element I3 to the interior Wall of the well, I have employed a lsodium silicate binding medium. This I have done by making a saturated solution of pure hydrated sodium silicate, or NazSiOa(9HaO), applying this to the installed winding, and curing by baking, to drive off water and moisture of combination, at a temperature slowly raised up to or beyond 600 degrees F. While a sodium silicate binding medium is broadly well known, I have found that its described use with the glass-insulated wire results in a structure wherein the wire becomes imbedded in, and united to the shell through, an almost-homogeneous sheath (formed apparently because of a large measure of amalgamation of the binding medium with the glass insulation, to which it has basically similar characteristics) which not only has the good heat-conducting properties lof sodium silicate, but at the same time has a particularly favorable mechanical Strength and recuring) for the winding before its insertion into the shell, thereby still retaining all of the described advantages of the treatment as to the element I3 considered apart from the shell.

The curing process for the sodium silicate binding medium applied to the element I3 in place in the shell is of course carried out before the final installation of the disc 6 and tightening of the sleeve 5. At its conclusion (or at the conclusion of a special de-hydrating heating of the device if that curing process is not carriedout). I permit the device to cool in a dry atmosphere; and then, without affording any opportunity for the re-acquisition oi moisture, I clamp the disc 6 by the sleeve 6 through gaskets I as above described, thus hermetica'lly sealing the interior of the device. Thereafter the sleeve 5 may be held against loosening movement by a pin 9 passed through the sleeve into biting engagement with the base portion 4. v

The choice of the material of the resistance wire Ilitself will of course be made according to the resistance-change requirements which it is desired that the device meet. To obtain a high degreeof sensitivity, however, I have very satisfactorily employed wire of a material commonly nickel, 17% cobait, 0.2%l manganese, and the balance iron. This alloy is specially calculated 4' for similarity o its thermal expansion coeiilcient with that of glass or similar materials; and it may be noted that the element I3 formed of it in the sheath I6 above described is therefore particularly free of temperature-induced Vmechanical strains. l This wire as commercially available neither is characterized by the full resistance-temperature coeiiicient of which it is capable, nor is its coeiiicient entirely stable. I have found that its resistance-temperature coeillcient may be raised and at the same time renderd perfectly stable, by suitably annealing the wire in an atmosphere of dry hydrogen-more specifically, by bringing it up in such an atmosphere in a furnace to atem- I perature over 900 degrees C., and preferably beportion of vsaid shell spaced from said base, said degrees (higher temperature tending to higher Y coeiiicient). With this treatment a coemcient yieldingthe following series of values is readily achieved: 81.6 ohms at -50 degrees C.; 100 ohms at 0 degrees;` 140.5 at 100 degrees; 173.2-at 180 degrees.

It will of course be understood that slight adjustments both of the mean absolute resistance order of .05 calorie per degree centigrade perv4 and of the resistance-temperature coeiiicient of l the device, as seen between the pins 8, may be eiected by suitable choice of the material and dimensions of the conductive leads I5 and of the precise length of resistance wire in the element I3.

Figure 5 illustrates a typical employment .of the device of earlier gures. Thel device appears herein in its entirety as 20, being connected between two terminals 2I and 22 of a triple-contact receptacle 2l. A battery or other current source B is connected between one of these terminals (22) and a third terminal 23. The terminals 2| and 22,'and therethrough the device 20, are connected to form one of the arms of a Wheatstone bridge, whose other arms are schematically shown as 25, 26 and 21. The terminals 22 and 23, and therethrough the battery B, are connected in one diagonal path across the bridge,

` while anindicating device or meter M is connected across the other diagonal path. The bridge being balanced as desired at some one temperature of the element I3 in the device 20,

the meter M will thereafter indicate the deviations from that temperature.

It will be understood that while in describingv my invention I have referred to various dimensional characteristics, these, excepting when indicated to be essential, are intended as exemplary only. Likewise while I have illustrated and de- I claim: Y ll. In aresistance bulb adapted for the electrical measurement `of the temperature of a uid medium and having a metallic supwrting base adapted to be secured ,in and thermally iniiuenced by an element of substantially different temperature from said medium: the combination of a thin shell secured to said base in thermally con-` ductive relationship thereto and extending from said base for immersion in said medium, and a temperature-variable resistance element within a portion of said shell spaced from said base, said shell constituting ,the principal thermal capacity veffectively associated with said element' and the principal path for heat conduction from said element to said base, and being formed of a -metallic material having a thermal conductivity at most of the order of .07 calorie per degree centigrade per square centimeter per centimeter per second.

2. In a resistance bulb adapted for the electrical measurement of the temperature of a fluid medium and having ametallic supporting base y adapted to besecured in and thermally iniiuenced by an-element of substantially different temper; ature from said medium: the combination of a shell constituting the principal thermal capacity effectively associated with said element and the principal path for heat conduction from said element to said base, and being formed of a metallic material having a thermal conductivity of the square centimeter per centimeter per second.

3. In a resistance bulb adapted for the electrif cal measurement of the temperature of a ii-uid medium andhaving a metallic supporting base adapted to be secured in and thermally' inuenced by an element of substantially diierent tempera-v ture from said medium: the combination of a metallicshell secured to said base in thermally conductive relationship thereto` and extending from said base for immersion in said medium, and a temperature-variable resistance element within a portion of said shell spaced from said base, said shellconstituting the principal thermal capacity electively associated with said element and the principal path Ior heat conduction from said element to said base, and the portion of said shell between said element and said base being characterized by a longitudinal thermal conduction at most of the order of .0015. calorie per de'- gree centigrade per second.

4. In a resistance bulb adapted for the electrical measurement of the temperature of a iiuid medium and having a metallic supporting base adapted to be secured in and thermally inii-uenced conductive relationship thereto and extending from said base for immersion in said medium,

` tively associated withA said element, and the portion of said shell between said element andsaid base being characterized by a longitudinal thermal conduction of the order of .0011 ycalorie per degree centigrade-.per second.

5. The combination. according to claim 1, wherein said shell constitutes at least two-thirds of tle thermal capacity associated. with said elemen 6. The combination according to claim l, wherein said shell constitutes at least two-thirds of the admittance to heat iiow between said element and said base.

7. In a resistance bulb adapted for the elecmedium and having a metallic supporting base adapted to be secured in and thermalLv innuenced .by an element of substantially different temperature from said medium: the vcombination of a thin shell secured to said base in thermally conductive relationship thereto and extending from said -base for immersion in said medium, and a temperature-variable resistance element within a portion of saidshell spaced from said base, said shell constituting the sole metallic support of said element and the principal thermal capacity eiiectively associated with said element. and being formed of a metallic alloy having a thermal condmtivity at most of the order oi .07 calorie trical measurement of the temperature of a fluid j per degree centigrade per square centimeter per centimeter Der second 8. In a resistance bulb adapted for' the electrical measurement oi' the temperature oi' a uid medium and having a metallic supporting base adapted to -be securedin and thermally influenced by an element of substantially different temperature from said medium: the combination of a metallic shell secured to said-base in thermally conductive relationship thereto and extending from said base for immersion in said medium, and a temperature-variable resistance element within a portion of said shell spaced from said base, said shell constituting the sole metallic support of said element and the principal thermal capacity effectively associated with said element. and being characterized, in its portion between said elementvand said base, by a longitudinal thermal conduction at most of the order of .0015 calorie per degree centigrade per second.

mmm G. my. 

